Method for preparing the viewing-screen structure of a cathode-ray tube

ABSTRACT

METHOD COMPRISES (A) PREHEATING TO ABOUT 50 TO 75* C. A VIEWING-SCREEN SUPPORT, A VIEWING SCREEN THEREON AND A LIGHT-RELFECTIVE METAL LAYER ON THE SCREEN, (B) DEPOSITING ON THE PREHEATED METAL LAYER A COATING OF AN ORGANIC, VOLATILIZABLE FILM-FORMING MATERIAL FROM AN AQUEOUS DISPERSION THEREOF, AND (C) DEPOSITION ON THE COATED PREHEATED METAL LAYER AN OVERCOATING OF CARBON PARTICLES FROM A BINDER-FREE SUSPENSION OF CARBON PARTICLES IN A LIQUID. THEN, THE SUPPORT AND STRUCTURES THEREON ARE BAKED IN AIR AT ABOUT 400 TO 450*C. TO REMOVE ORGANIC AND VOLATILE MATTER.

United States Patent 3,703,401 METHOD FOR PREPARING THE VIEWING- STRUCTURE OF A CATHODE-RAY Samuel Broughton Deal, Lancaster, and Donald Walter Bartch, Columbia, Pa, assiguors to RCA Corporation No Drawing. Filed Dec. 28, 1970, Ser. No. 102,148 Int. Cl. H01j 3/06; B4411 5/00 US. Cl. 117-335 CP 9 Claims ABSTRACT OF THE DISCLOSURE Method comprises (a) preheating to about 50 to 75 C. a viewing-screen support, a viewing screen thereon and a light-reflective metal layer on the screen, -(b) depositing on the preheated metal layer a coating of an organic, volatilizable film-forming material from an aqueous dispersion thereof, and (c) depositing on the coated preheated metal layer an overcoating of carbon particles from a binder-free suspension of carbon particles in a liquid. Then, the support and structures thereon are baked in air at about 400 to 450 C. to remove organic and volatile matter.

BACKGROUND OF THE INVENTION This invention relates to a novel method for preparing the viewing-screen structure of a cathode-ray tube.

One type of cathode-ray tube that is used for television displays is referred to as a shadow-mask tube. This tube is comprised of an evacuated envelope having a viewing window, a viewing screen comprised of a mosaic of phosphor areas (usually dots) of dilierent emission colors supported on the inner surface of the viewing window, a shadow mask having an array of apertures therein in register with the phosphor areas mounted in the tube in adjacent spaced relation with the window, and means for projecting one or more (usually three) electron beams towards the screen for selectively exciting the phosphor areas of the mosaic.

In operating a shadow-mask tube, the electron beams are made to scan a raster in a fixed pattern. As the beams are made to scan, they are either intercepted by the mask or they pass through the mask apertures and excite the desired phosphor areas. The energy in the intercepted electron beams heats the mask and may cause the mask to become distorted, which may adversely affect the position of the beams which pass through the mask apertures. Some of the heat in the mask is removed by radiation back to a black coating on the funnel of the tube. Normally, the viewing-screen structure includes a thin layer of a highly reflective metal, usually aluminum, which reflects heat that is radiated forward towards the screen.

US. Pat. No. 3,392,297 to J. W. Schwartz suggests applying to the reflective layer an overcoating of a heatabsorptive material that is also inert and can be evaporated and vapor deposited in a vacuum. Lithium nitride, boron carbide and nickel oxide are examples that are given for such materials. This suggestion is good in principle and may achieve its object. However, these materials are not easily or cheaply evaporated and vapor deposited in a vacuum in the factory. Also, these evaporable materials may migrate during the subsequent operation of the tube.

SUMMARY OF THE INVENTION In the novel method of making a cathode-ray tube, a heat-absorptive overcoating of carbon particles is deposited upon the reflective metal layer. Carbon particles are not evaporable at ordinary tube processing temperatures or under normal operating conditions of the tube. Preferably, the carbon particles are applied by spraying fll an aqueous suspension of carbon particles. Aqueous spraying is cheaper and easy to carry out in the factory, and is substantially free of fire hazards.

In order to produce consistently good screens, however, the novel method includes (1) preheating the screen structure to about 50 to C. in order that the carbon coating does not run when it is applied; (2) applying a sealer coating or barrier layer to the reflective metal layer prior to applying the carbon coating in order to prevent the suspension of carbon particles from penetrating into the phosphor areas of the screen where they can absorb the emitted light and reduce the brightness of the screen; and (3) omitting the use of a binder from the suspension of carbon particles because the presence of a binder increases the amount of oxidation of the carbon particles during the subsequent baking steps of the tubemanufacturing processes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of the novel method are applied to the manufacture of a color-television picture tube of the type described in US. Pat. 3,423,621 to Martin R. Royce. The preferred embodiments of the novel method start with a faceplate panel which is subsequent to the aluminizing of the viewing screen. The panel has a phosphor mosaic viewing screen deposited on the inner surface thereof, a volatilizable film deposited thereon and a light-reflective layer of aluminum metal about 2500 angstroms thick deposited on the film. Processes for preparing this intermediate structure are known, as exemplified by US. Pat. No. 3,067,055 to Theodore A. Saulnier, Jr. and Pat. No. 3,582,389 to T. A. Saulnier, Ir., and include depositing a viewing screen upon the inner surface of viewing window or other support, depositing a volatilizable film upon the viewing screen, and depositing a reflective metal layer upon the film. Since these processes are described in detail elsewhere they need not be redescribed here.

Example 1 An example for practicing the novel method by hand is now described. The panel and intermediate structure thereon are placed in an oven that is preheated to 6515 C. and kept there for about 15 minutes until the panel is at about the oven temperature. The panel is removed from the oven and the panel seal lands and the inner sidewalls of the panel including the mask-mounting studs are masked as with masking tape to about the mold match line but leaving the entire viewing area unmasked. Then, with the panel still preheated, an aqueous dispersion of a volatilizable film-forming material is sprayed upon the unmasked aluminum metal layer. A preferred dispersion is prepared by mixing 250 milliliters of an aqueous acrylic resin emulsion (containing about 46 weight percent solids) with 750 milliliters deionized water and 25 milliliters Monastral dye solution (20% solids in water). A preferred acrylic resin emulsion is Rhoplex AC-33 marketed by Rohm and Haas Company, Philadelphia, Pa, which is believed to be constituted principally of ethyl acrylate copolymerized with minor amounts of acrylic and methacrylic monomers and polymers. The dye is included in the dispersion to simplify inspection of the resultant coating after it has been deposited. The spraying is conducted for about 2 to 5 minutes with an air spray gun operating at about 20 pounds per square inch pressure, and includes about ten passes of the spray across the surface. The sprayed material dries in less than a minute due in part to the heat in the preheated panel.

Then, with the panel still preheated, and the mask in place, a binder-free aqueous suspension of graphite and carbon black is sprayed upon the unmasked portions of the coated metal layer. A preferred suspension is prepared by mixing about 1000 milliliters of a weight percent aqueous dispersion of colloidal graphite with about 1330 milliliters deionized water and about 1000 milliliters of a 5 Weight percent aqueous dispersion of carbon black, such as Vulcan XC-72R carbon black marketed by Cabot Corporation, Boston, Massachusetts, and containing about 0.5 weight percent dispersing agent such as Marasperse CB and 0.1 weight percent wetting agent such as Brij 35. The spraying is conducted for about 2 to 5 minutes with an air spray gun operating at about pounds per square inch and includes about twenty passes of the spray across the surface. The sprayed material dries in less than a minute due in part to the heat in the preheated panel.

The mask is removed, and the coated panel is now processed in the usual way. This includes the usual step of baking the panel in air at about 400 to 450 C. to remove by vaporization and oxidation the volatile and organic matter in the structure. In this last baking step, the film and coating of volatilizable material underlying and overlying the aluminum metal layer, the binders in the mosaic viewing screen, and all of the dispersing agents and wetting agents in the structure are removed. After baking, the structure includes an aluminum metal reflective layer on the phosphor mosaic viewing screen and a heat absorbent carbon-and-graphite overcoating adhered upon the aluminum layer.

Example 2 An example for practicing the novel method with automatic operating machinery is substantially the same as in xample 1 except in the following respects. The panel is preheated with radiant heat to 60 to 70 C. in about 3 minutes. Then, the panel surface is masked with a stencil of predetermined design. The preferred dispersion is prepared by mixing 747.5 grams of the acrylic resin solution (AC-33) with about 250 grams deionized water and about 2.5 grams Rhodamine B Extra Red dye powder. This dispersion is sprayed with automatic operating machinery for about seconds using an air spray gun operating at about 60 pounds per square inch pressure. The carbon suspension also is sprayed with automatic operating machinery for about 30 seconds using an air spray gun operating at about 60 pounds per square inch pressure.

GENERAL CONSIDERATIONS There are many variations that may be made to the preferred embodiment that fall within the scope of the novel method. The carbon overcoating is applied to the reflective metal layer after the metal layer is deposited but before the structure is baked to remove the organic film used to impart a shiny surface to the metal layer. The structure has a substantial number of pores to permit the escape of gases during subsequent baking steps.

Either graphite or amorphous carbon or a combination of the two may be used for the carbon overcoating. Amorphous carbon may be in the form of lamp black, carbon black or other forms prepared from the incomplete burning of carbon-bearing materials. The graphite may be synthetic or natural. It has been observed that graphite particles are more resistant to oxidation and tend less to penetrate the viewing screen that the amorphous carbon particles. Amorphous carbon particles produce layers that are more heat absorbent and are less resistant to electron penetration. A mixture of the two types of carbon is preferred.

The particle size of the carbon particles is not critical but is preferably colloidal in size to facilitate the preparation and maintenance of a suitable suspension. The carbon may be suspended in any liquid vehicle such as toluene or xylene. However, it is preferred to disperse the carbon in water. When carbon particles are dispersed in water, it has been found desirable to include wetting and Cit dispersing agents for the purpose of producing a stable suspension. Also, it has been found desirable to omit binders for the particles from the suspension. When binders have been included, it has been found that the carbon particles may oxidize excessively during the subsequent baking step, thereby making the process control more diflicult.

The sealer coating or barrier layer may be of any volatilizable film-forming material which will perform the function of preventing carbon particles from entering the pores in the aluminum metal layer and passing to the phosphor mosaic or viewing screen. Some suitable materials for this purpose are acrylic copolymers such as Rhoplex B74, B83, B85, C72 and D70, all marketed by Rohm and Haas Company, Philadelphia, Pa. Another suitable material is a polystyrene emulsion such as 40-201 Synthemul marketed by Reichhold Chemicals, Inc., White Plains, N.Y. The use of a barrier layer or sealer coating provides a means for preventing the carbon from penetrating through the aluminum metal layer into the phosphor mosiac when the carbon suspension is applied. Such penetration could result in staining, smudginess, and reduced visual brightness of the phosphor screen.

Preheating the panel and intermediate structure assists in drying the coatings that are subsequently applied thereto. The omission of preheating results in running of the coatings. Solvent-based carbon formulations have been made which do not need the preheating step. However, water-based systems of both the carbon formulation and the sealer formulation require the preheating step. The water-based systems are preferable because of the better safety aspects and because of reduced equipment costs. Preheating temperatures are in the range of about to 75 C. and preferably about C.

The sealer coating and the carbon overcoating may be applied by any convenient process. The preferred process is by air spraying because of its low cost and great convenience. The sealer coating is applied as the thinnest layer which will provide the function of blocking carbon particles from penetrating into the phosphor mosaic. The carbon overcoating is applied in a thickness of about 2,000 to 3,000 angstroms. This should be controlled because the thickness affects the penetration of the electron beam which eventually can excite the phosphor mosaic. Excessively thick carbon layers should be avoided since they reduce the brightness of the screen. The aluminum metal layer is also preferably about 2,000 to 3,000 angstroms thick so that the combination of aluminum metal layer and carbon layer is between 4,000 and 6,000 angstroms thick. Ordinarily the aluminum metal layer itself is about 4,000 to 6,000 angstroms thick.

The process is preferably applied to shadow-mask tubes for the purpose of providing a heat-absorbing layer on the aluminum reflective layer of the screen structure. The method may be applied to other cathode-ray tube types for the purpose of depositing carbon in any of its forms to the aluminum. metal layer. For example, the method may be applied to providing a carbon layer upon a metal reflective layer to reduce secondary emission and electron scattering. The use of a carbon layer for reducing secondary emission and scattering is described in U.S. Pats. 2,878,411 to L. W. Alvarez and in 3,475,639 to J. 'P. Driffort et al.

We claim:

1. In a method for making a cathode-ray tube having a viewing-screen support, a viewing screen thereon, and a light-reflective metal layer on said screen, the steps subsequent to producing said metal layer comprising:

(a) preheating said support with said screen and metal layer thereon to about 50 to C.,

(b) depositing upon said preheated metal layer a coating of an organic volatilizable film-forming material, said coating being deposited from an aqueous dispersion thereof,

() and depositing upon the coated metal layer an overcoating of carbon particles, said overcoating being deposited from a suspension of carbon particles in a liquid, said suspension being substantially free of organic and inorganic binders.

2. The method defined in claim 1 wherein said filmforming material is an acrylic copolymer.

3. The method defined in .claim 1 wherein said suspension is comprised of amorphous carbon particles in an aqueous vehicle.

4. The method defined in claim 1 wherein said suspension is comprised of graphite particles in an aqueous vehicle.

5. The method defined in claim 1 wherein said suspension is comprised of a mixture of amorphous carbon particles and graphite particles in an aqueous vehicle.

6. In a method of making a cathode-ray tube having a viewing-screen support, a phosphor mosaic viewing screen thereon, a light-reflective metal layer on said screen and a shadow mask mounted in said tube in adgiacent spaced relation with said screen, the steps comprising:

(a) depositing said viewing screen upon said support,

(b) depositing a volatilizable film upon said viewing screen,

(0) depositing said light-reflective metal layer upon said film,

(d) preheating said support and metal layer thereon to about 50 to 75 C.,

(e) then, while still preheated, depositing on said metal layer a coating of a volatilizable film-forming material, said coating being deposited from an aqueous dispersion thereof,

(f) then, while still preheated, depositing upon the coated metal layer an overcoating of carbon particles, said overcoating being deposited from a suspension thereof that is substantially free of organic and inorganic binders,

(g) and then heating said support and the structures thereon at about 400 to 450 C, in air until said film is volatilized.

7. The method defined in claim 6 wherein step (e) comprises spraying an aqueous dispersion of an acrylic copolymer upon said metal layer and then permitting said sprayed material to dry.

8. The method defined in claim 6 wherein step (f) comprises spraying an aqueous suspension of graphite and amorphous carbon particles upon said coated metal layer and then permitting said sprayed material to dry.

9. The method defined in claim 6 wherein said metal layer and said overcoating of carbon particles are each about 2,000 to 3,000 angstroms thick.

References Cited UNITED STATES PATENTS 3,475,639 10/1969 Drifiort et a1. 31392 R 2,597,617 5/1952 Campbell 11733.5 CP 3,392,297, 7/1968 Schwartz 3l3-92 B 2,951,773 9/1960 Helle et al. 1l7-2l1 ALFRED L. LEAVITT, Primary Examiner M. F. ESPOSITO, Assistant Examiner US. Cl. X.R.

117---16, 33.5 CM, R, 38, 45, 47 H, 71 R, 216 

